Conventionally, the method for sterilizing or completely sterilizing various microorganisms such as bacteria or viruses can be broadly divided into two types, i.e., a physical method (mechanical method) using heat, pressure, or the like, and a chemical method using chemical agents. The physical method includes a high-pressure steam sterilization method (autoclaved sterilization), a gamma sterilization method, an electron beam sterilization method, and the like. The chemical method includes an ethylene oxide gas (EOG) sterilization method, a reduced-pressure hydrogen peroxide plasma sterilization method, and the like.
The physical method is mainly used for complete sterilization of medical equipment. However, since an object to be sterilized is exposed to an extreme physical condition in many cases, the type of object to be sterilized is limited. For example, the autoclave is not applicable to a plastic product that is easily affected by heat, and it is not desirable to sterilize, using gamma rays, materials, precision equipment, or the like that are easily degraded by ultraviolet rays or the like. In addition, a large-scaled apparatus is required in most cases except for using the autoclave, which, in many cases, makes it difficult to choose a place to install the apparatus.
In the case of the chemical method, the chemical agents to be used may exert a harmful influence on the human body. Therefore, a process for securely rendering the agent residue harmless is necessary, which, as a result, requires more cost and time. Particularly, guidance is provided to restrict the use of EOG because it has acute toxicity and a mutagenic property. Especially, when chemical agents are used, complete sterilization in a liquid is difficult. Even if such sterilization is realized, high-concentration chemical agents remain in the liquid as a result. It becomes, then, extremely difficult to detoxify the sterilization agents dissolved in the liquid, i.e., substantially impossible by conventional technologies.
In recent years, on the other hand, investigations for a sterilization method using plasma have been in progress. The plasma is an expression referring to a state of substance in addition to solid, liquid, and gas. Atoms turn into a plasma state consisting of ions and electrons, and acquire a high chemical activity at a high temperature of about 10,000° C. or higher. The plasma is used as a light source represented by a fluorescent lamp or processing used in the semiconductor industry.
Provided as an example of the sterilization method using plasma is the hydrogen peroxide plasma sterilization method (HLPS) as described above. In the HLPS method, a pressure in a chamber is reduced to, for example, as low as 0.3 Torr; hydrogen peroxide is injected and diffused; and high-frequency discharges (10 eV, 13.56 MHz, 400 W) by means of air are performed. Thereafter, clean air is blown into the chamber to bring the pressure therein back to the atmospheric pressure. It is said that, according to the HLPS method, sterilization is performed by radicals (OH.) or the like produced by an oxidation action of hydrogen peroxide and the plasma discharges (Non-patent Document 1: “Features and Problems on the Plasma Sterilization and Future Prospect of the New Sterilization using Low-temperature-plasma”, by Kaoru Tamazawa, Bokin Bobai Vol. 32, No. 1, pp. 13-30).
Further, JP-A-2004-290612 discloses a sterilization method which uses hydrogen peroxide as a sterilization chemical agent and combines the same with plasma.
It is also proposed to ignite plasma in a chamber maintained as a vacuum environment after exhausting gas, and sterilize a dried object (Non-patent Document 2: “Plasma Sterilization”, by Masaaki Nagatsu, J. Plasma Fusion Res., vol. 83, No. 7 (2007) 601-606). Non-patent Document 2 discloses sterilizing medical equipment such as a knife, a scalpel, and a tube disposed in a non-woven fabric that allows gas to pass therethrough but prevents microorganisms from passing therethrough.
According to the conventional sterilization method utilizing a combination of hydrogen peroxide and plasma, what is currently performed is the sterilization by using hydrogen peroxide that, per se, is a powerful microbicide, and thereafter the plasma is used to decompose and detoxify the hydrogen peroxide.
In addition to this fact, a pressure vessel is required to use plasma under a vacuum environment in a low pressure. The sterilization is performed only in such a pressure vessel, which casts a lot of restrictions in performing the sterilization.
In recent years, atmospheric pressure plasma has been drawing a lot of attention. The conventional plasma is generated, in often cases, under a low pressure, and it is difficult to use it under an ordinary environment. When the plasma is generated under a high pressure such as an atmospheric pressure, the plasma thus generated tends to become a thermal plasma represented by arc plasma used for arc-welding, because particles that are ionized to become plasma, through frequent collisions with neutral gas particles, reach almost a thermal equilibrium state accompanied by an increase in the temperature of neutral gas component. On the other hand, non-equilibrium plasma is attracting attention. The non-equilibrium plasma is chemically active because it has a sufficiently high electron temperature despite a low neutral gas temperature and formed by creating a non-equilibrium state while ingeniously contriving a way to prevent a thermal relaxation state from being generated. The non-equilibrium plasma is sometimes called low-temperature plasma because the neutral gas temperature is about a room temperature, which is significantly low as compared with the electron temperature.
Further, as for the use of plasma, JP-A-2008-010373 proposes a device which emits plasma onto an object to be processed made of a conductive material for processing. According to JP-A-2008-010373, generated plasma is derived effectively and increased by applying a bias voltage between the object to be processed and a discharge electrode, so that the object to be processed is exposed to the plasma efficiently for processing.
As discussed above, in conventional technologies, sterilization using plasma has not yet reached the stage of practical use. If plasma, in particular, the non-equilibrium plasma described above can be used for sterilization, the practical value thereof is supposed to be extremely high because restrictions for performing sterilization are largely reduced.
Further, if ions or radicals generated by plasma can be supplied to a liquid efficiently, such ions or radicals can be practically used to sterilize microorganisms present in the liquid, and so on.
Meanwhile, according to the processing method described in JP-A-2008-010373, a DC bias voltage or AC bias voltage is used as a bias voltage to be applied between the object to be processed and the discharge electrode. It is deemed that, in JP-A-2008-010373, the bias voltage is applied to cause discharges also between the plasma and the object to be processed, and the plasma is directly emitted onto the object to be processed, leading to the improvement in processing efficiency.